The present invention relates to a confocal microscope and also relates to a relay optical system used mainly in a confocal microscope. More particularly, the present invention relates to a confocal unit used mainly for observation of an image of a specimen by imaging and displaying a confocal image with an image pickup device and a display device, which can be removably attached to a microscope body to construct a confocal microscope that is compact in size as a whole and suffers a minimal loss of brightness.
There has heretofore been publicly known a confocal microscope in which a scanning disc for confocal microscopy, such as a Nipkow disc, which is positioned in conjugate relation to a specimen, is rotated at high speed to scan the specimen with illuminating light, thereby obtaining a confocal image of the specimen. Such a confocal microscope is described in detail, for example, in U.S. Pat. No. 4,927,254 and commercially available from various manufacturers.
Japanese Patent Application Unexamined Publication (KOKAI) Number (hereinafter referred to as "JP(A)") 10-26735 (1998) discloses a confocal unit that can be removably attached to a microscope to construct a confocal microscope system. In this confocal unit, a laser is used as a light source to illuminate a specimen through a microlens array corresponding to pinholes on a scanning disc, thereby obtaining a bright confocal image. In the confocal unit, the optical axis of a C-mount and a photographing device lie on the same straight line.
Optical systems for relaying an image formed by an objective lens or the like include one type of relay optical system that comprises two lens units each having a positive focal length and that has a relatively wide spacing between the two lens units. Examples of such a relay optical system are disclosed in JP(A) 61-210312 (1986) and 09-33804 (1997).
Confocal microscopes commercially available from various manufacturers at present are designed so that a confocal image can be visually observed. To allow visual observation, the height of the eyepiece portion from the desk surface (i.e. eye point) must be within certain limits. That is, if the eye point is excessively high, it becomes difficult to look through the microscope. Accordingly, the relay optical system usually adopts a folded optical path arrangement or the like in which the optical path is folded in a plane perpendicular to the optical axis of the objective lens, thereby keeping the eye point from becoming high. However, a large number of mirrors are used to form such a folded optical path. The number of reflections by mirrors may reach as many as ten- or more reflections in total from the light source to the observer's eye, which will result in a large loss of brightness. In confocal microscopes, to which the present invention relates, loss of brightness is a serious problem in that it is difficult to perform observation because the image is unfavorably dark unless a high-intensity light source is used. Furthermore, because the arrangement becomes complicated, the number of parts used increases, and thus the cost is unfavorably high.
The confocal unit disclosed in JP(A) 10-26735 (1998) is designed to be removably attached to a microscope to construct a confocal microscope system and therefore has a relatively simplified arrangement. However, illuminating light must be applied from above the microlens array owing to the principle of the device. Accordingly, viewing light must be drawn out from the optical path halfway through a beam splitter or the like, and it is necessary to use at least several mirror members. Consequently, loss of brightness cannot be avoided.
Even if the number of mirror members used is minimized, if the overall length of the optical system increases, the unit itself lengthens and becomes unstable when attached to a microscope, and it becomes inconvenient to install an image pickup device. Therefore, it is necessary to make the overall length of the optical system as short as possible. However, in the confocal unit, the specimen needs to be illuminated through the scanning disc. Therefore, it is necessary to ensure a space sufficient to place a prism member or a mirror member for introducing illuminating light between the scanning disc and the relay optical system. From this point of view, relay optical systems disclosed in JP(A) 61-210312 (1986) and 09-33804 (1997) are similar in arrangement to the relay optical system according to the present invention.
In a relay optical system that comprises two lens units each having a positive focal length as disclosed in JP(A) 61-210312 (1986) and 09-33804 (1997), the distance L from the intermediate image position to the relay image position is assumed to be 2.times.(f.sub.1 +f.sub.2), where f.sub.1 is the focal length of the front lens unit, and f.sub.2 is the focal length of the rear lens unit.
JP(A) 61-210312 (1986) adopts an arrangement in which the specimen-side lens surface in the second lens unit is a strong concave surface directed toward the object side in order to place a chromatic dispersion prism between the lens final surface and the image plane. By using a strong concave surface directed toward the object side as the specimen-side lens surface in the second lens unit, JP(A) 61-210312 (1986) intends to shift the principal point position toward the image plane to thereby increase the distance between the second lens unit and the image plane. With this arrangement, however, although the distance between the second lens unit and the image plane can be increased, because the distance between the first lens unit and the second lens unit is also increased, the distance between the intermediate image and the first lens unit inevitably reduces undesirably in a limited space "L". Accordingly, with this arrangement, the spacing between the scanning disc, which is placed at the intermediate image position, and the first lens unit reduced, so that it becomes difficult to place a prism member or a mirror member for introducing illuminating light. If the focal length of each lens unit is increased, it becomes possible to place a prism member or a mirror member for introducing illuminating light. However, "L" also lengthens if the focal length of each lens unit is increased. Consequently, it becomes impossible to construct the optical system in a compact form.
In the relay optical system disclosed in JP(A) 09-33804 (1997), the first lens unit comprises a positive meniscus lens having a concave surface directed toward the intermediate image side. If the concave surface is directed toward the intermediate image side, the principal point position shifts closer to the image plane side. Therefore, it is impossible to ensure a sufficiently wide spacing between the intermediate image position and the first lens unit. Accordingly, it becomes difficult to place a prism member or a mirror member for introducing illuminating light. In this case also, if the focal length of each lens unit is increased, it becomes possible to place a prism member or a mirror member for introducing illuminating light. However, "L" also lengthens if the focal length of each lens unit is increased. Consequently, it becomes impossible to construct a compact optical system.